Band-pass amplifier



DBC. 12, R Al BRADEN 1,938,620

Filed Aug. 23, 1929 2 Sheets-Sheet l WCM/IVE .5763041 FME-76) INVENTORRENE A. BRADEN ATTORNEY Dec.

12, 1933. R. A. BRADr-:N

BAND- PASS AMPLIFIER Filed Aug. 23, 19.29

2 sheets-sheet 2` INVENTOR RENE A BRADEN ATTORNEY Patented Dec. 12, 1933PATENT ori-*ICE BAND-PASS AMPLIFIER Rene A. Braden, New York, N. Y.,assignor to Radio Corporation of America, a corporation of DelawareApplication August 23, 1929. Serial No. 387,836

12 Claims.

My present invention relates to amplifiers, and, more particularly, toband-pass amplifiers employing novel coupling means.

The typical band-pass amplier circuit of present day practice possessesresonance curves, withr various values of coupling between the tunedcircuits, which have pronounced, undesirable features. One particularlydisadvantagecus characteristic of this type of circuit is that 10. theWidth of accepted band is limited by the depression at the center' ofthe band, which depression becomes deeper as the coupling is increased.

Again, at any frequency, either at resonance l5Y or off resonance, thereis a particular value of coupling which gives to the secondary circuitthe maximum current that can flow therein. Thus, the optimum coupling issmall for the resonance frequency, and becomes larger as the frequencyis farther from resonance. The value of coupling and band width at whichthe center depression begins depends, of course, on the amount of ohmicresistance in the tuned circuits. It is, therefore, customary to insertresistance in theV circuits when a greater band widthr than usual isdesired. rIhis has the desired result of increasing the band width.However, the amplification is reduced in proportion to the amount ofresistance added.

Now, I have discovered a type of circuit in which the effective value ofthe coupling varies according to the frequency, being small at theresonance frequency of the tuned circuits, and relatively large forfrequencies off resonance. Thus, every frequency within certain limitswhich impressed in the circuit encounters the proper value of couplingto amplify it properly. All frequencies within these limits are,therefore, amplified equally. Moreover, no resistance need be added tothe circuits, and the amplification obtainable from a wide-bandampliiier is thereby, according to my invention, as great as that of anarrow-band ampliiier.

45 Accordingly, it is one of the main objects of my present invention toprovide an improved coupling means for band-pass amplifiers by means ofwhich it is possible to construct a bandpass ampliiier passing a widerband than is 50. passed by an amplifier employing the usual, present daycircuit, without the use of a large number of tuned circuits in eachamplifier stage, and without damping the tuned circuits.

Another important object'of the invention is '35 to provide a method of,and means for, coupling two tuned circuits so that the coupling isrelatively small at the resonantl frequency of the circuits, and largerat frequencies, higher and lower than the resonant frequency.

Another object of the invention is to provide a band-pass amplierconsisting of two tuned circuits, and means for coupling the twocircuits in such a way that the co-eicient of coupling is substantiallyproportional at any frequency to the difference between that frequencyand the resonant frequency of the tuned circuits.

Still other objects of the invention are to iJnprove generally theelciency of band-pass amplifier couplings.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims, the inventionitself, however, as to both its organizationl and method of operationwill best be understood by reference to the following description takenin connection with the drawings in which I' have indicateddiagrammatically several circuit organizations whereby my invention maybe carried into effect.

In the drawings,

Fig. l schematically shows a typical band-pass amplifier circuitemployed in present practice,

Fig. 2 shows resonance curves for the circuit in Fig. l' with variousvalues of coupling,

Fig. 3 shows a `circuit embodying my novel 85' form of coupling,

Figs. 4 and 5 show modified forms of the invention,

Fig. 6 graphically shows the coupling coefficient of the circuit in Fig.5,

Figs. 7 and 8 show'additional modified forms of the invention.

Referring to the accompanying drawings in which like characters ofreference indicate the same elements in the different views, Fig. 1illus- 95 trates an amplifier circuit which is typical of all band-passcircuits which have been used heretofore. The incoming signal energy isimpressed across the input terminals of the tube 1, the latter being ofthe screen grid type. The output circuit of the tube is tuned by thevariable capacity 2 andinductance 3.

The input circuit of the screen grid tube'9 is tuned by the inductance 7and variable capacity 8, the input circuit being coupled to the'output105 circuit 2, 3 by coupling coils l0, 11, as atM.` The output from thetube 9 is then impressed upon a' succeeding stage and utilized in anywell known manner.

In Fig. 2, I have shown resonance curves for the circuit in Fig. l, withvarious values of coupling M between the two tuned circuits. Theabscissa is Frequency, and the ordinate Secrondary current, curve Aillustrating the effect of loose coupling, while curve E portrays theresult of tight coupling, the curves between designating increasingvalues of coupling.

The particular feature of this circuit, which it is desired toillustrate by means of the resonant curves of the Fig. 2, is that thewidth of the accepted band is limited by the depression at the centre ofthe band, which depression becomes deeper as the value of the coupling Mis increased. It will, additionally, benoted that at any frequency,either at resonance or off resonance, there is a particular value ofcoupling which gives to the secondary circuit the maximum current thatcan flow therein; and that the optimum coupling is small for theresonance frequency and becomes larger as the frequency is farther fromresonance.

The value of coupling and band width at which the depression beginsdepends upon the amount of resistance in the tuned circuits. It iscustomary, therefore, to insert resistance in the tuned circuits when agreater band width than usual is desired. This allows the coup to beincreased, thus increasing the band width, but the amplification isreduced in proportion to the amount of resistance which is inserted intothe tuned circuits.

My invention, as explained heretofore, and illustrated in Figs. 3 to 8inclusive, involves a circuit in which the effective value of thecoupling varies according to the frequency, being small at the resonancefrequency of the tunen1I circuits, and relatively large for frequenciesofi resonance, Every frequency within said limits., which is impressedon the circuit, encounters the proper value of coupling to amplify itproperly, and all frequencies Within these limits are t. ereforeamplified equally. Moreover, no resistance need be added to thecircuits, and the ainpiication obtainable from a wide-band amplifier istherefore as great as that of a. narrow-band amplifier.

In Fig. 3, there is shown an amplifier circuit embodying a couplingdesigned according to my invention. The incoming signal energy isiinpressed across the input terminals of the screen grid tube 1, theoutput circuit of the tube being shown as iixedly tuned by the capacity2 and the inductance 3. It is to be understood that a vacuum tube of thetriode or tetrode type may be employed in the circuit. The outputcircuit ofthe tube l is coupled to the tuned input circuit of the screengrid tube 9 by a coupling circuit to be shortly described in detail.

The input circuit of the tube 9 is shown as Iixedly tuned by thecapacity 8 and the inductance '7, and the output from the tube 9impressed upon a succeeding stage and utilised in any well-knownfashion. The coupling circuit comprises an inductance coil 4 coupledinductively to the coil 3, and an inductance coil 6 coupled inductivelyto the inductance '7, the coils 4 and 6 being connected in series witheach other.

A net-work 5 of resistances and reactances is inserted in series withthe coils 4 and 6, the network including an inductance L1 and aresistance R1 in series with each other, and a capacity C in series witha resistance Rz, the latter two elements being in shunt with the seriesconnection of resistance andinductance. It is tc be noted that coils 6and 4 are similar to each other, coil 7 is similar to coil 3, andcapacity 8 is similar to the capacity 2.

In operation, the net-work 5, consisting of the inductance L1, thecapacity C', and the resistances Ri and R2, is tuned to the mid-bandfrequency or" the band of frequencies which is to be ainplified, theoutput circuit of tube 1 and the input circuit of tube 9 beingmaintained xedly tuned to the same mid-band frequency. It is to beobserved that the last named two circuits correspend to the primary andsecondary circuits of Fig. i, and that they are coupled by the circuit4, 5 and 6,

The effective coupling between the circuits 2, 3, and 7, 8 depends uponthe coupling between the coils S and 4, and the coupling between thecoils 6 and 7, and also on the impedance of the circuit'4, 5, 6, thecoupling being diminished as the impedance of this latter circuit isincreased. The net-work circuit 5, being resonant to the frequency towhich the primary and secondary circuits are tuned, puts a highimpedance in the coupling circuit at that frequency, and reduces thecoupling to a small value. At irequencies which are higher and lowerthan the resonant frequency, the impedance of the net work 5, issmaller, and the coupling is greater.

Adjustment of the circuit may be effected by first adjusting thecoupling between coils 3 and 4 and between 6 and '7, so that with thenet-work 5 short circuited, the two peaks of the resonance curve areiai' enough apart to give the desired band width. Then, the net-work 5is inserted, and the circuit elements L1, C', Ri and R2 are adjusted sothat the impedance at parallel resonance has the value which gives theproper degree oi coupling at the mid-band frequency of the acceptedband, and so that the impedance oi net-work 5 changes at the proper ratewith increasing or decreasing frequency.

In Fig. 4, I have shown a modification of the circuit shown in Fig. 3.It is to be pointed out at this time that the reference numerals in thisfigure and the following figures denote the same circuit elements as inFigs. 1 and 3. In Fig. 4, the coupling circuit impedance 5 consists ofthe coupled inductances L1 and L2 (the inductances being coupled as atM), the condenser C and the resistance R', the condenser, resistance andinductance La being in series with one another.

rihe circuit is otherwise exactly the same as shown in Fig. 3, but is amore convenient circuit than that shown in Fig. 3 from the standpoint ofease of adjustment, it being pointed out that its koperation is exactlythe same as in the casc of Fig. 3. The capacities 2, 8 and C' arevariable in this modification. In Fig. 5 the coupling circuit impedance5 consists of two serif-s resonant circuits L1, C1, and L2, C2, theseries resonant circuits being connected in shunt. These series resonantcircuits are adjusted so that one is resonant at approximately the upperfrequency limit of the band, and the other is resonant at the lowerfrequency limit, while the two in combination are in parallel resonanceat the mid-band frequency of the accepted band. In 'this modification,the capacities 2 and 8 are iixed.

In Fig. 6 I have graphically shown the coupling coefficient produced bythe coupling circuit in Fig. 5. In this iigure frequencies, as abcissae,are plotted against coupling coeicients, as ordinates, the accepted bandbeing shown as pos- 150 sessing a width denoted between the dottedvertical lines.

Fig. 7 shows an amplifier Vcircuit including fixedly tuned circuits 2, 3and 7, 8, in which there are provided coupling link circuits,'denoted by5 and 5", respectively. Circuit 5 is tuned to one frequency limit, andcoupling circuit to the other limit of the accepted band.r Link circuit5 contains an inductance L' and a capacity C' in series, While linkcircuit 5" con-V tains an inductance L" anda capacity C'I in series.rThe circuit operates in a manner similar to the circuits alreadydescribed, this particular modification producing low coupling at mid-Each impedance comprises a series resonant ciri cuit tuned to the samefrequency as the two tuned circuits 2, 3 and 7, 8, .that is to say, tothe mid-band frequency. The impedance 12 is made up of an inductance Lin series with a capacity C', while the impedance 13 comprises aninductance L in Yseries with the capacity C".A

At the mid-band frequency, the coupling coils 1G and ll are shuntedeffectively by resistances R and R", and the effective coupling isreduced to a small value, the amount of reduction being determined bythe magnitudes of R and R". At other frequencies above or below themidban i frequency, the series resonant circuits 12 and 13 have higherimpedances, and the effective coupling is increased. The rate at whichthe coupiing increases as the frequency. departs from the mid-bandvalue, is determined by thev ratio of if: C and L: C".

The coupling systems disclosed herein are particularly applicable to asuperheterodyne receiver, or to a transmitter, in which the circuits areleft xedly tuned to one frequency. The coupling circuits must be tuned,as well the stage input and output circuits, and there are, therefore,actually three orfour tuned circuits in each stage. Obviously, in thecase of a tuned radio frequency receiver, might prove cumbersome.However, in the case of a superheterodyne, or a transmitter fortelevision purposes, the present arrangement might prove to be verydesirable because of the extremely wide band which is utilized in atelevision system.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention by no means limited to the particular organizationsshown and described, but that many modifications in the circuitarrangements, as weil as in the apparatus employed, may be made withoutdeparting from the scope of my invention as set forth in the appendedclaims.

What I claim is:

1. A band-pass amplifier comprising two oscillatory circuits resonant tothe same frequency, and a network resonant to said frequency forcoupling the two circuits, said network comprising inductive andcapacitative reactances of such relative magnitudes; that thecoefficient of coupling between said circuits is substantiallyproportional at any frequency to the difference between-that frequencyand the resonant frequency of the tunedv circuits whereby the couplingbetween the circuits is a minimum at said resonant frequency. 2. Aband-pass amplifier stage comprising input and output circuits, twotuned circuits resonant to a desired frequency, a link circuit couplingthe tuned circuits, and a parallel resonant circuit inserted in serieswith the link circuit, tuned to the same frequency as the tunedcircuits, said coupling link circuit consisting of inductive andcapacitative reactances of such relative magnitudes as to provideminimum coupling between the two tuned circuits at the desired`frequency and maximumcoupling at frequencies 01T resonance.

3. A band-pass amplifier stage comprising two tuned circuits, two linkcircuits, each independently coupling the tuned circuits, each linkcircuit tuned by series inductance and capacity to one end of thefrequency band which is accepted by the circuits, both said linkcircuits being in parallel resonance at the mid-band frequency. i

4. A band-pass amplifier comprising two tuned circuits resonant to amid-band frequency, a link circuit coupling the two tuned circuits, an

Vinduetance coil inseries with the link circuit,

and aseries resonant circuit coupled to the coil in the link circuit andbeing tuned to said midvband frequency of the accepted frequency bandwhereby the coupling between the two tuned eircuits is a minimum at saidmid-band frequency.

5. A band-pass amplifier circuit comprising auf" pair of tuned circuitsresonant to the same frequency, a network resonant to the saidfrequencyfor coupling said circuits, said coupling network inciuding inductiveand capacitative imn pedances ofY such relative magnitudes that theeffective value of thek coupling varies according to the frequency, thevalue of the coupling being small at the resonance frequency of thetuned circuits and relativelyy large for frequenciesfoff resonance. i,

6. In combination, an amplifier having a tuned output circuit, a secondamplifier having a tuned input circuit, both said tuned circuits beingresonant to a desired mid-band frequency, and a network coupling saidtuned circuits, said netg-i` work being resonant to the said frequencyand including capacitative and inductive reactances of such relativevalues that its impedance at mid-band frequency and the rate of changeof its impedance for frequencies off the mid-band frequency is such thatall frequencies within the band are substantially uniformly amplified.

7. In combination, an amplifier having a tuned output circuit, a secondamplifier having a tuned input circuit, both said tuned circuits beingresonant to a desired mid-band frequency, and a network coupling saidtuned circuits, said network being resonant to the said frequency andincluding capacitative and inductive reactances of such relative valuesthat its impedance at mid-band frequency and the rate of change of itsimpedance for frequencies ofi the midband frequency is such that allfrequencies withn in the band are substantially uniformly amplifled,said network including a series resonant, path tuned to the upperfrequency limit of the band including said mid-band frequency, and asecond series resonant path tuned to the lower frequency limit of thesaid band.

8. In combination, a source of alternating cur,-

rent energy to be amplified, -a tuned-circuit cou-k circuit, a secondamplifier having a -tuned -input circuit, both 'said tuned circuitsbeing resonant'V to a desired mid-band frequency; anda network couplingsaid tuned circuits, said network being resonant to the said frequencyand including condensers and inductances of such values that* itsimpedance at mid-band frequency and the circuits, saidimpedancepathbeing resonant to rate of change of its impedance for frequencies offlthe mid-band frequency is such that allfre quencies within the band aresubstantially uni` formly amplified.

9. In combination, a source of alternatingcur-v rent energy to beamplified, a tuned circuit coupled thereto, an amplifier having a .tunedoutput circuit, a second amplifier having a tuned input circuit, bothsaid tuned circuits being resonant to a desired mid-band frequency, anda network coupling said tuned circuits, said network being resonant tothe said frequency and including condensers and inductances of suchvalues that its impedance at mid-band frequency and the rate of changeof its impedance for frequencies off the mid-band frequency is such thatall frequencies within the band are substantially uniformly amplified,said network including a series resonant path tuned to the upperfrequency limit of the band including said mid-band frequency, and asecond series resonant path tuned to the lower frequency limit of thesaid band.

10. In combination, a source of alternating current energy of a wideband of frequencies to be amplified, a tuned circuit coupled thereto, anamplifier tube having a tuned input circuit, both said tuned circuitsbeing resonant to the midband frequency cf said wide band offrequencies, and an impedance path coupling 'said tuned circuits, saidimpedance path being resonant to said mid-band frequency and includingcapacities and inductances of magnitudes such that the coupling betweensaid tuned circuits is a minimum at said mid-band frequency, and isincreasingly greater for frequencies within said band but off themid-band frequency whereby 'beamplifiei -av tuned circuit coupledthereto, an

lincreasingly greater Vfor Afrequencies within said"allffrequenciesvwithin `the saidvsid'e band are substantially uniformlyamplified.

if `1'1'.=In"combination, a source of alternating c'urr'entlenergy ofav-wide band of frequencies to amplifier tube having atuned inputcircuit, both said-tuned circuits being resonant to themidbandvfrequency ofl'said'wide band of frequen- "cies, and an impedancepath coupling said tuned said midbandvfrequency and including capacitiesandinductances of magnitudes such that the 'coupling between said tunedcircuits is a minimumatv said mid-band frequency, and is band butofffthe mid-band frequency whereby all frequencies within the` said sideband are .substantially uniformly amplified,V said imped- .ance vpathincluding a series resonant circuit `tuned to the-upper frequency limitof said band 1 and asecond series resonant circuit tuned to the"cessive' screen grid iainplier tubes, means for 100 coupling saidsource to the input electrodes of the first tube, means for connectingthe output electrodes of the secondl tube to a succeeding stage, a coilconnected across the output electrodes of the first tube, and a coilconnected across the'input electrodes of the second tube, a condenseracross each of said coils for resonating the coils to the mid-bandfrequency of a desired frequency band to be amplified, a couplingnetwork coupled between said coils, said network including a pluralityof inductances and condensers having relative magnitudes sufficient toresonate said network to said mid-band frequency and to cause thecoupling coefficient between said coils to be a minimum at said midbandfrequency and to be increasingly greater for frequencies within saiddesired band but off the mid-band frequency.

RENE A. BRADEN.

